I'm going to do a playthrough challenge "can you beat Pokemon HeartGold with only 1 Mareep", and I'm stuck on what my plan is to beat Whitney. Any pointers would help :)
There is precious little you can do. Miltank has better stats than Mareep in every category except Special Attack, which it doesn't use anyways. If the Mareep is female, it's at least able to avoid the effects of Attract. If it's male, you have to suffer from infatuation. You could attempt to Thunder Wave it and pray for parahax, Growl to make it less of a menace, EV train to help it resists attacks, or overlevel Mareep. All of these options are unreliable or time-consuming, and grinding out levels at the Daycare is likely your best bet, even if it is tedious. You could alternatively try to Toxic stall it, but it would take forever, and Mareep would probably be KOed.
Hope I helped!